Polymethylmethacrylate bone cements have been in use in medicine for decades for permanent mechanical fixation of total joint endoprostheses. These are based on powder-liquid systems, whereby it is customary to use methylmethacrylate as monomer. A general overview is provided, e.g., in K.-D. Kühn, Knochenzemente für die Endoprothetik: ein aktueller Vergleich der physikalischen and chemischen Eigenschaften handelsüblicher PMMA-Zemente, Springer-Verlag Berlin Heidelberg New York, 2001.
U.S. Pat. No. 8,536,243 B2 describes a powder-gel bone cement system. This concerns a modification of the conventional powder liquid bone cements. The powdered component consists of a polymer, an initiator such as dibenzoylperoxide, and, if applicable, a radiopaquer and hydroxyl apatite. The gel-like component comprises an acrylic monomer such as methylmethacrylate, a radical inhibitor such as hydroquinone, an activator such as N,N-dimethyl-p-toluidine, and a polymer that is dissolved in the acrylic monomer and has a mean molecular weight above 1,000,000 g/mol.
Recently, polymethylmethacrylate bone cements based on the use of cement pastes have also been proposed. DE 102007052116 A1 relates to a one-component bone cement. DE 102007050762 B3 and DE 102008030312 A1 describe two-component bone cements made of two cement pastes that are stored separately in suitable cartridges. The bone cements each contain at least one monomer, suitable polymeric ingredients, and a redox initiator system. In most cases, methylmethacrylate is used as the monomer.
The redox initiator systems used in this context usually consist of peroxides, accelerators and, if applicable, suitable reducing agents. Radicals are formed only if all components of the redox initiator system act in concert. For this reason, the components of the redox initiator system are arranged appropriately in the separate cement pastes such that they cannot trigger a radical polymerisation.
The cement pastes are stable during storage. Only when the two cement pastes are mixed to produce a cement dough, the components of the redox initiator system, previously stored separately in the two pastes, react with each other to form radicals which trigger the radical polymerisation of the monomer that is present. The radical polymerisation then leads to the formation of polymers while consuming the monomer, whereby the cement dough is cured.
It is customary to use static mixers for mixing the cement pastes and to attach them to 2-component cartridges for this purpose. When the two cement pastes are extruded from the cartridges, the two cement pastes are pushed through a static mixer. As a result, the processes of extruding and mixing proceed concurrently. Mixing the cement pastes in the static mixer requires a high extrusion force since the pressure drop at the mixing elements in the static mixer is very high. It is therefore necessary to use powerful pneumatic or mechanical extrusion devices to dispense and mix the cement pastes. Said pneumatic or mechanical extrusion devices are elaborate from a technical point of view and expensive.
An inexpensive option are the manually-operated extrusion guns, which are customary with polymethylmethacrylate bone cements based on powder-liquid systems, which are suitable for said cements, but are not sufficiently powerful for extruding and mixing bone cement pastes through the use of static mixers.
In conventional 2-component cartridges, the volume ratio of paste A to paste B often is 1:1, 1:2, and 1:10. The more the volumes of the pastes to be mixed through the use of static mixers differ, the more difficult it is to generate a homogeneously mix paste. For this reason, very many mixing spirals are needed for large volumes. The larger the number of mixing spirals needed, the larger is the pressure drop in the static mixer during the mixing process. This means that the pastes need to be pressed through the static mixer by a very large force. Due to the nature of manually-operated extrusion devices, the maximally possible extrusion force is limited.
The bone cements usually contain radiopaquer for the cement used in the body to be visible in a radiograph. Common radiopaquers, such as zirconium dioxide and barium sulfate, possess a high density though. Accordingly, if these are present in pastes there is a risk that the radiopaquers sediment which might adversely affect the quality of the cement.
It was the object of the invention to provide a two-component bone cement that overcomes the disadvantages of the prior art described above. Specifically, it shall be feasible to manually mix the two components without using static mixers in order to obtain a cement dough that is tack-free and capable of plastic deformation immediately. Moreover, the nature of the polymethylmethacrylate bone cement to be developed should be such that the radiopaquer is prevented from sedimenting. Any interaction of the monomer with possibly added pharmaceutical agents should also be excluded. Moreover, the components should be simple and inexpensive to manufacture.
The underlying idea of the invention is to develop a polymethylmethacrylate bone cement that allows a pasty component to be manually mixed with a powdered component in appropriate manner such that a self-curing cement dough is generated that can be extruded and applied by means of common, manually-operated cementing devices. The idea is to let the mixing process and the extrusion process of the polymethylmethacrylate bone cement take place one after the other, which is in contrast to two-component polymethylmethacrylate bone cements made up of two pastes, in which the mixing and the application take place concurrently while the two pastes are being extruded through a static mixer. The mixing of two pastes requires the input of a relative large amount of energy. The application of two-component polymethylmethacrylate bone cements therefore necessitates extrusion devices with a high extrusion force.
Surprisingly, it has been evident that it is feasible, by manual mixing of a pasty component, as defined below, with a second powdered component, as defined below, to produce a tack-free plastically deformable bone cement dough that is self-curing by means of radical polymerisation.